Turkey agreed to support a new U.S.-led effort to train and equip moderate Syrian rebels, American officials said, as Islamic State advances on a Syrian border city led to warnings of a massacre.

The deal to train rebels doesn’t provide any Turkish relief for the besieged city of Kobani on Turkey’s border, which was in danger of falling to the extremist group despite the U.S.-led coalition’s most intense bombardment since it began airstrikes in Syriaseveral weeks ago. (More: Turkey Agrees to Train Syrian Opposition on Home...